The invention concerns a method for operation of a four-stroke reciprocating internal combustion engine with a homogeneous lean base mixture consisting of air, fuel and retained exhaust, as well as with compression ignition and direct fuel injection into a combustion chamber whose volume is varied cyclically and which is fillable through at least one inlet device with fresh gas and whose combustion exhausts can be exhausted through at least one outlet device, at least partially.
Reciprocating internal combustion engines during compression ignition of homogeneous lean mixtures offer the possibility of limited NOx formation with high thermal efficiency. A prerequisite for this is achievement of an optimal ignition temperature in which optimal beginning and running of a self-maintaining combustion is established.
At insufficient temperature of the mixture during compression combustion begins too late and proves incomplete. At unduly high temperatures, steep pressure increases with knocking combustion develop owing to the then self-accelerating combustion.
The optimal ignition temperature can be achieved by variation of compression and/or variation of retention of exhausts from the previous cycle. Variation of these quantities is very complicated with a completely variable valve control system. Moreover, during variation of compression via a variable intake, there is the drawback that the admissible compression and thus filling must be reduced with increasing output so that engine output is limited. Another output limit results from operation with the lean mixture. Using high percentage of exhaust, even a stoichiometric mixture can be burned with compression ignition with avoidance of steep pressure increases and NOx emission, but with the drawback of reduced gas filling and thus power losses.
These shortcomings also apply to DE-A 195 19 663 in which a method is described for operation of an internal combustion engine with compression ignition. In this case a homogeneous and lean air/fuel mixture generated with external mixture formation is compressed in a first stage to close to the ignition limit. In a second stage, an additional mixture of the same fuel is finely atomized and injected into the combustion chamber, avoiding wall contact. The late injected fuel forms a mixture cloud, which is ignited, since its ignition limit lies below the compression temperature achieved in the first stage owing to the higher fuel content.
The underlying task of the invention is to devise a method of the generic type mentioned in the preamble of claim 1 through which low NO.sub.x emission and high efficiency are achieved with the least possible design expense.
The task is solved by a method with the features of claim 1.
By operation with compression ignition at partial load, which predominates in vehicle operation, limited NO.sub.x emission and low consumption are ensured. Effective control of the combustion start and process by mechanically controlled exhaust retention then requires comparatively limited expense. High power is achieved by spark-ignition engine operation at high partial load and full load. This type of load requirement, however, accounts for only a limited part of the total operating time so that the NO.sub.x emission and fuel consumption occurring in this case play a subordinate role. The cost required for this is kept within the usual limits for spark-ignition engines.
The exhaust mass required to control combustion during compression ignition forms in the combustion chamber by combustion of the fuel and air from the supplied fresh mixture. The energy liberated during combustion is taken off by the crankshaft by expansion to the maximum combustion chamber volume. A discharge cross section is then opened and exhaust is discharged by reducing the volume. During the exhaust process, on reduction of the combustion chamber volume, the exhaust valve closes and retains part of the exhaust. The amount of exhaust is compressed again to minimal combustion chamber volume and thus kept thermally active. The necessary mass of exhaust can be retained in the combustion chamber only if the intake valve after compression of the exhaust opens at a volume greater than the volume at which the exhaust valve was closed before compression.
The retained exhaust comes from combustion with an air excess. An amount of fuel that enters into an incipient chemical reaction with the residual air present, which cannot react completely because of the rapid increase in volume, but forms a significant number of chemically active radicals and thus easily ignites the then supplied fresh gas mass during subsequent compression, can be injected into the already expanding, compressed exhaust by means of the prescribed internal mixture formation.
It is not necessary for chemical activation to control the mass of retained exhaust as precisely as for compression ignition without activation. By chemical activation (radical formation), the effect of thermal activation and the ignition-relevant effect of exhaust retention are intensified severalfold. A smaller but distinct valve undercut can therefore be prescribed.
The valve undercut is simply accomplished mechanically with a second cam shape. Switching between an SI engine cam with valve overlap and one with valve undercut is easily accomplished with the so-called VTEC system. The engine can be operated in the full load range and in the region of the upper partial load with the second, adjustable cam form, which corresponds to an SI engine design. To this we add the usual elements in SI engine operation, like constant compression, inlet throttle valve and spark ignition for the homogeneous, stoichiometric mixture. The time cross section for valve lifting with the valve undercut should amount to roughly half the time cross section at full load.
The necessary retained amount of exhaust depends on the activation conditions, which are set as a function of the load and speed according to the conditions of injection and the beginning of chemical reaction. Control of the amount of exhaust via the time cross section is no longer possible with a fixed mechanical cam shape. The required amount of exhaust is not set via the duration of opening or the size of the time window of valve opening, but via the pressure difference between the combustion chamber and the exhaust throttle behind the discharge valve. The exhaust sensor flap required for this purpose controls the pressure difference between the combustion chamber and the exhaust train before the valve closes again for compression of exhaust retention.
The time cross section of the inlet device is much smaller than that of the outlet device. The smaller time cross section of the inlet device is compensated with respect to mass flow rate by the larger pressure difference being adjusted.
Since a valve overlap is not present, but rather a valve undercut in operation with exhaust throttle valves, the pressure state in the exhaust manifold cannot affect the state before the inlet valve. The two gas dynamic regions before and after the combustion chamber remain separated. Load control of operation occurs via adjustment of the exhaust throttle valve. This determines the maximum exhausted mass from the combustion chamber and thus the charging mass taken in for the next combustion stroke.
The exhaust throttle valve can be mounted at the end of the common exhaust manifold or directly behind each outlet device of each individual cylinder. The basic principle remains identical for both systems. An overlap of the exhaust mass from an individual cylinder into the others is not to be expected, since damming up does not occur simultaneously with a significant pressure difference at the relatively high rate of change of the combustion chamber volume.
The exhaust is retained by the exhaust sensor flap by constricting the flow cross section to a higher pressure than the ambient pressure. After the narrowest cross section, the medium is expanded to the surrounding pressure conditions and the density drops. If the exhaust throttle valve is provided with a continuously expanded cross section function over the flow length after the smallest cross section in the direction of flow, the density of the retained exhaust can be changed with avoidance of outflow loss or an expansion jolt. The outflow loss of a noncontinuous pressure and density change over the flowing medium is to be avoided.